1. Field of the Invention
This invention relates to InGaAs/InP type PIN photodiodes.
In optical communication, photodiodes are frequently used as photo-detecting elements. The requirements for such photodiodes are that they have high sensitivity and low dark current and further that they can readily be manufactured.
2. Description of the Prior Art
The structure of a conventional InGaAs/InP type PIN photodiode is first described by reference to FIG. 2.
This photodiode has a multi-layered structure formed by epitaxially growing on an n-InP substrate 21 and n-InP buffer layer 22, an n-InGaAs photo-detecting layer 23 and an n-InP window layer 24.
A Zn diffusion region 25 is further provided in the center portion of the n-InP window layer 24 whereby this portion is changed from n-type to p-type. The Zn diffusion region 25 extends to the n-InGaAs photo-detecting layer 23 to form a p-n junction 40.
Further provided on the surface of the Zn diffusion region 25 is a ring-like p-electrode 27 in which a window is provided to allow the incidence of light, the window being coated with an AR (antireflection)-film coating 28.
An n-electrode 26 is also provided by the vaccum evaporation process on the opposite end surface of the InP substrate 21.
Light enters through the AR-film coating 28 into the interior of the element. The window layer 24 consists of InP, band gaps of which are larger than those of InGaAs, and is rather thin. Accordingly, the amount of light absorbed at the layer 24 is relatively small.
Since the portions corresponding to the Zn diffusion region 25 is of p-type, the InP window layer 24 and InGaAs photo-detecting layer 23 may also be referred to as p-InP window layer 29 and p-InGaAs photo-detecting layer 30, respectively.
With this arrangement, light absorbed in the InGaAs photo-detecting layers 30, 23. If the energy of the light is larger than the band gap, pairs of electrons and holes are excited, and these pairs are accelerated under the influence of the electric field of the p-n junction 40.
The electrons drift to the n-electrode 26 through the n-InGaAs photo-detecting layer 23, the n-InP buffer layer 22 and the n-InP substrate 21.
The reason for the provision of the window 24 is as follows:
Minority carriers generated in the InGaAs photo-detecting layer 23 cannot pass over hetero-barrier 31 of the heteroplane 31 between the InGaAs layer 23 and InP layer 24. Therefore, the minority carriers cannot reach the surface of the element and thus surface recombination can be prevented. Moreover, the possibility of recombination at the heteroplane is also low. Thus, recombination of the minority carriers does not occur even in the vicinity of the heteroplane. It is, therefore, possible to efficiently take out the minority carriers generated in the photo-detecting layer. In this manner, the band barrier 31 of the heteroplane has an important function. That is why the window layer 24 is necessary.
The structure shown in FIG. 2 is arranged to introduce light from the window layer 29 of the p-type region, but there is another conventional structure as shown in FIG. 3 wherein light is introduced from the n-type substrate 41.
In the conventional example of FIG. 3, an n-InGaAs photo-detecting layer 32 is epitaxially grown on an n-InP substrate 41. A Zn diffusion region 33 is then formed on the n-InGaAs photo-detecting layer 32 whereby the region 33 changes from n-type to p-type InGaAs. Further, a p-electrode 34 is provided by the vacuum evaporation process on an upper surface of the Zn diffusion region 33.
Similarly, on the bottom surface of the n-InP substrate 41, a ring-like n-electrode 35 is provided by the vacuum evaporation process and it is provided therein with a light incidence surface having an AR coating 36.
The incident light enters from the bottom surface through the n-InP substrate 41 and the InGaAs photo-detecting layer 32 to a p-n junction area 37 to excite pairs of electrons and holes in that area. The pairs of electrons and holes are then accelerated and separated under the influence of the electric field of the p-n junction area 37. The electron drifts from the n-InGaAs photo-detecting layer 32 through the n-InP substrate 40 to the n-electrode 35 to generate external current.
The holes also drift through the p-InGaAs photo-detecting layer 33 to the p-electrode 34 in the vicinity of which the holes are recombined with electrons. Electrons are supplied from the p-electrode 34 to generate external current.